1. Field of the Invention
This invention relates to a fluorescent display device, and more particularly to a lead for an electrode in a fluorescent display device which is capable of being positively mounted on a substrate by soldering.
2. Description of the Prior Art
Typically, a lead frame of a fluorescent display device is made of 426 alloy an alloy of 42% nickel, 6% chromium with the remainder being iron) which has substantially the same coefficient of thermal expansion as that of frit glass used for hermetically sealing a casing on a substrate and good conformability with the frit glass, and formed into such a complicated shape as indicated by reference characters LF in FIG. 1 by etching a plate of 426 alloy. The lead frame thus formed is then subjected to an oxidation treatment to form a layer of chromium oxide (Cr.sub.2 O.sub.3 : chromium dioxide) thereon. The chromium oxide layer has good conformability with frit glass, which is adhesive and is used as a pigment for glass as well. Thus, the layer may improve adhesion between the lead frame and the sealing portion of the casing, to thereby provide satisfied airtightness.
The lead frame, as shown in FIG. 1, serves to hold control electrodes G provided as desired and is connected to electrodes such as anodes, cathodes, control electrodes and the like to act as lead for the electrodes. However, the external lead portion of the electrode lead does not materially have good conformability with solder, because it is made of 426 alloy. Accordingly, it is highly difficult to mount a fluorescent display device through the external lead on a substrate by soldering. Such a defect is aggravated when the chromium oxide layer is formed on the surface of the lead. In order to avoid such a disadvantage, it is required to carry out solder coating obtained by previously applying solder to the external lead portion from which the chromium oxide layer has been peeled.
The removal of the chromium oxide layer is carried out by a mechanical method using a brush or the like or a chemical method. The mechanical method is exclusively used, because the chemical method has an important defect that it is highly troublesome and difficult to treat a waste liquid containing Cr.
Further, the height or thickness of the external lead portion of the electrode lead is apt to be large. More particularly, the diversification of display function of a fluorescent display device increases the number of leads and each of the lead is obliged to have a reduced width. This results in the thickness of the lead wire being necessarily increased in order to ensure the strength and reliability. Also, as shown in FIGS. 2(A)-(E), it is generally required that etching of the lead frame using a resist R to form the external lead portion is excessively carried out in light of the control of dimensions of the extenal lead portion so that both side surfaces of the external lead portion each may have a concave shape, because a failure in the control of width l of the external lead portion causes the contact between the external lead portions of the adjacent leads. This renders the removal of the chromium oxide layer from the concave side surfaces of the external lead portion by a mechanical method using a brush or the like substantially impossible, since the brush or the like fails to evenly contact with the concave side surfaces.
Such failure in removal of the chromium oxide layer, as shown in FIG. 3, hinders the application of solder coating onto the side surfaces of the external lead portion. Accordingly, the bonding between the side surfaces of the external lead portion and a substrate PC may not be carried out during the mounting of a fluorescent display device on the substrate PC by soldering, as shown in FIG. 4, resulting in soldering failure which is known as a tunnel solder phenomenon. This causes an important disadvantage that the bonding between the external lead portion and the substrate is made with weak bond strength and a small bond area, to thereby often incur the electrical contact failure therebetween and fail in the mounting of a fluorescent display device on the substrate with good reliability.